M&I Week 2 Flashcards

1
Q

Two Distinct Functions of Antibodies

A

Conferred by two distinct portions of the antibody. Variable region interacts with antigen. Constant region interacts with other components of the immune system, which mediate different effector functions of antibodies

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Variable Region of the Antibody

A

Highly diverse among different antibodies and interacts with the antigen. Each antibody contains two that can bind with antigen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Number of Variable Regions in an Antibody

A

2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Constant Region of an Antibody and Isotypes

A

Each antibody has only one constant region and it interacts with other components of the immune system, which mediate distinct effector functions of antibodies. Comes in five different forms (IgM, IgG, IgA, and IgE), each specialized to activate distinct components of the immune system and thus elicit distinct effector functions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Number of Constant Regions in an Antibody

A

1 & 5 isotypes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

B-Cell Differentiation

A

Naive B Cells start off with producing surface immunoglobulins of the IgM and IgD varieties (co-expressed). Upon antigen recognition and B Cell activation, these B cells might differentiate into plasma cells that secrete antibodies of the IgM variety. Can occur in a T Cell independent manner, but most require T Cell help

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Somatic Hypermutation

A

Some activated B Cells that receive T Cell help will undergo somatic hypermutation that results in the introduction of point mutations within the variable region of the antibody, which ultimately results in the development of antibodies with increased affinity for antigen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Affinity Maturation of the Antibody Response

A

Somatic Hypermutation can lead to the development of antibodies with increased affinity for antigen, resulting in “affinity maturation of the antibody response”. Requires intimate interactions of B Cells with T Cells and of B Cells with antigens trapped by follicular dendritic cells in germinal centers of lymphoid organs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Germinal Center Reaction

A

Affinity maturation requires intimate interactions of B Cells with T Cells and B Cells with antigens trapped by follicular dendritic cells in germinal centers of lymphoid organs, a process referred to as the germinal center reaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Class Switch Recombination

A

When B Cells switch their antibody class from IgM (and IgD) to one of the other classes, without altering their antigen-binding specificity, in a genetic process referred to as class switch recombination

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Differentiation into Plasma Cells vs. Differentiation into Memory B Cells

A

Plasma cell differentiation is a terminal process, whereas memory B Cells can be reactivated by specific antigen and undergo further somatic hypermutation and class switch recombination

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Shape of Antibodies

A

Y-shaped, the structure of the cell surface expressed form of an immunoglobulin is identical to that of the secreted form, except for a short hydrophobic portion in the carboxy terminus of the heavy chain that anchors the protein to the membrane

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Number of Name of Polypeptide Chains (Antibodies)

A

Antibodies contain two types of polypeptide chains, termed a heavy (H) and light (L) chain. Each antibody contains two heavy chains and two light chains

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How Are Heavy Chains Linked to Each Other

A

Disulfide bonds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How Are Heavy Chains Linked to Light Chains

A

Disulfide bonds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Two Types of Light Chains

A

Kappa and Lambda

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Isotypic Exclusion

A

A given antibody contains either kappa or gamma light chains, but never one of each

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What Determines the Class of an Antibody

A

The heavy chain of the antibody

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Five Different Heavy Chains

A

Mu, delta, gamma, alpha, and episilon, which corresponds to the five classes of antibodies (IgM, IgD, IgA, IgE). Can also contain subtypes for each heavy chain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Two Isotypes of Antibody That Can Polymerize

A

IgM assembles as a pentamer and IgA can assemble as a dimer

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

J-Chain in Antibody Polymerization

A

Multimerization of IgM and IgA monomers is facilitated by interaction of a polypeptide called a J-Chain, with cysteine residues in the “tailpiece” of IgM and IgA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Functions of Multimer Antibodies (IgM and IgA)

A

IgM is the first antibody class to be synthesized and its pentameric form permits high avidity interactions with antigen; IgA plays an important role in mucosal immunity and its transport across mucosal membranes requires dimerization (usually present as a monomer in the blood and lymph)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

H and L Chains Contain Multiple Domains of What Length and What Structure

A

110 amino acids in length which fold into a structure referred to as a Beta-Barrel (two anti-parallel Beta-sheets connected by a disulfide bond)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Immunoglobulin Fold

A

The particular beta-barrel structure adopted by an immunoglobulin domain. This type of fold is quite popular among proteins of the immune system (as well as the nervous system)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Which Termini of Heavy and Light Chains Are Highly Variable

A

The amino-termini of both the heavy and light chains of immunoglobulins are highly variable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Variable Domains of Heavy and Light Chains

A

Variability predominantly contained within the N-terminus immunoglobulin domains of the heavy and light chains, which are referred to as the variable domains of the heavy and light chains, Vh and Vl, and together form the variable region of an antibody

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Constant Domains of the Heavy and Light Chains

A

The constant domains of the heavy and light chains (Ch and Cl) together form the constant domain of the antibody molecule. Individual domains within the constant region of the heavy chain are referred to as Ch1, Ch2, and so on

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Papain

A

Protease that cleaves antibodies just above the disulfide bonds that connect the two heavy chains together, resulting in three fragments: two fragments called Fab fragments (Fragment antigen binding), are identical and can each interact with antigen, and the third fragment, called the Fc fragment (Fragment cystallizable) mediates the effector functions of an antibody. Differences between distinct antibody classes are largely contained in the Fc portion of an antibody

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Pepsin

A

This protease cleaves antibodies multiple times below the disulfide bonds that connect the two heavy chains, resulting in one main fragment, the F(ab’)2 fragment, which contains who Fab fragments (and thus two antigen-binding sites) linked by disulfide bonds, and multiple fragments derived from the Fc portion of an antibody

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Recombinant DNA technology and Antibody Structure

A

Permits one to modify the structure of an antibody molecule, for example to attach the constant region of a human Ig-gamma chain gene to the variable region of a murine Ig-gamma chain, in order to convey human effector functions to an antibody generated in mice against a human therapeutic antigen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Hinge Region of an Antibody and Flexibility

A

Hinge region linked the two Fab fragments and the Fc portion. Other portions of the antibody molecule, such as the region between the variable and constant domains are also flexible. Permits antibodies to interact with multiple identical antigens that are spaced at a variable distance on a surface

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What Biochemical Structures Can Antibodies Interact With

A

Proteins, carbohydrates, nucleic acids, lipids, and small molecules

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Antigens

A

Any substance that can be recognized by an antibody. Most but not all antigens can induce an antigen specific immune response when introduced into (by immunization or vaccination, typically in the context of an adjuvant that induces activation of the innate immune system; AlOH) animals or human subjects

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

Immunogens

A

Antigens that can induce an antigen-specific immune response when introduced (by immunization or vaccination, typically in the context of an adjuvant that induces activation of the innate immune system) into animals or human subjects

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Haptens

A

Antigens such as small organic chemicals (e.g trinitrophenol) can be recognized by antibodies but cannot elicit an antibody response when injected by themselves. Can be converted to immunogens by conjugating them to a carrier proteins

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

Hypervariable Regions

A

With regard to immunoglobulin heavy and light chains and stretches of amino acids, where variability is concentrated: Three highly variable regions, which can be identified in the variable region of both the heavy and light chains, separated from less variable regions (framework regions). Clustered within three loops at the outer edge of the immunoglobulin domain, forming the antigen-binding site

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

Framework Regions

A

Less variable regions and separated from the hypervariable regions, of which there are four in both the heavy and light chains

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

Complementary Determining Regions (CDRs)

A

Refers to the six hypervariable regions, as they form a surface complementary to the antigen. The third complementarity determining regions (CDR3), which have the highest variability, are most critical for interaction with antigen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

Epitope

A

The particular area recognized within the antigen by the antibody

40
Q

Continuous/Linear vs. Discontinuous/Conformational Epitope

A

Antibodies can bind to antigens in their native or denatured form. Epitopes within a native protein can be contained either within the same stretch of amino acids (continuous or linear epitope) or more comonly within different parts of the polypeptide chain that are brought together in the folded protein (discontinuous or conformational epitope)

41
Q

Forces That Bind Antibody/Antigen Interactions

A

Non-covalent forces: electrostatic, hydrogen bonding, Van der Waals interactions and hydrophobic interactions

42
Q

Bruton’s Tyrosine Kinase & X-Linked Agammaglobulinemia (XLA)

A

Individuals with a genetic mutation in an X-linked kinase called Bruton’s tyrosine kinase (BTK), involved in B Cell signaling, develop X-linked agammaglobulinemia (XLA). Lack B-Cells and suffer from recurrent bacterial and viral infections. Antigen binds membrane bound Ig (cross-linking RTK). Noral Btk phosphorylates PLC-gamma which increases cystolic calcium levels which increases NFAT TF and DAG which activates PKC and then NF-kB

43
Q

How Do Antibodies Enter Most Tissues

A

Antibodies enter most tissues via simple diffusion

44
Q

How are IgA Antibodies Transported

A

Can be transported across epithelia for deivery to mucosal sites such as the lung and intestine

45
Q

First Antibodies to be Generated

A

First antibodies to be generated are of the IgM class

46
Q

Shape, Presentation, Function of IgM Antibodies

A

Form pentamers with a total of 10 antibody-binding sites, capable of binding with multivalent antigens in blood, at lower concentrations in lymph, and at very low levels in tissues. Particularly adept at activating the complement system

47
Q

IgG Antibody

A

Activated during a second immune response, which can diffuse into tissues. Will often have undergone affinity maturation and will therefore have increased affinity for antigens. Most important antibody class in blood and tissues and confers immunity to neonates because it can cross the placenta (half-life is 2-3 weeks), so it provides protection in the neonate for up to 6 months. Also effective in activating complement and opsonizing pathogens for uptake via phagocytes

48
Q

IgA Antibody

A

Activated during a second immune response, which can diffuse into tissues. Will often have undergone affinity maturation and will therefore have increased affinity for antigens. Dimeric form most common found in secretions, including mucosal secretions in the lungs, gut, saliva, and tear glands, and in breast milk

49
Q

IgE Antibody

A

Acitvated during a second immune response, which can diffuse into tissues. Will often have undergone affinity maturation and will therefore have increased affinity for antigens. Plays a key role in allergic reactions and are present at very low levels in blood and tissues, but are typically bound with receptors on mast cells found beneath the skin and mucosal surfaces, and around blood vessels in connective tissue

50
Q

Five Different Effector Functions Mediated by Antibodies

A

Neutralize pathogens and toxins
Activate complement
Opsonize pathogens
Promote antibody-dependent cell-mediated cytotoxicity (ADCC) in NK Cells
Activate Mast cells, basophils, and eosinophils
The latter three functions involve interactions of antibodies with Fc receptors expressed by distinct cell types

51
Q

Neutralization of Pathogens and Toxins by Antibodies

A

Pathogens often bind with host cells via specific receptors (called adhesins for bacteria or attachment proteins for viruses) and toxins (derived from bacteria, insects, or snakes), similarly interact with specific host receptors. Can be interrupted with antibodies of the IgG or IgA classes, thus neutralizing the pathogen or toxin. Vaccines often designed to elicit neutralizing antibodies

52
Q

Complement Activation by Antibodies

A

When bound with antigens on pathogen surfaces (IgM and IgG) antibodies undergo a conformational change in their Fc portion that permits binding of C1q of the classical complement pathway. Activation of complement in this manner can lead to activaiton of the MAC, induction of inflammation, opsonization of pathogens, and clearance of immune complexes

53
Q

Opsonization By Antibodies

A

When bound with antigens on pathogen surfaces IgG antibodies can bind with Fc-gamma receptors on phagocytes. Engagement and cross-linking of multiple Fc-gamma receptors on phagocytes lead to the ingestion and destruction of pathogens

54
Q

Induction of ADCC in NK Cells By Antibodies

A

Pathogen derived proteins can be expressed at the surface of infected cells and become targets for recognition by antibodies. Likewise, autoimmune responses often result in generation of antibodies directed against autologous surface antigens. Cells bound by IgG antibodies can be recognized by Fc-gamma receptors expressed by NK cells and trigger target cell destruction

55
Q

Activation of Mast Cells, Basophils, and Eosinophils By Antibodies

A

Mast cells, basophils, and activated eosinophils express Fc-epsilon receptors that can interact with IgE antibodies. IgE antibodies can bind with these receptors even in the absence of antigen. Engagement of multivalent antigen with IgE on mast cells then leads to cross-linking of the Fc-epsilon receptors and rapid release of inflammatory mediators such as histamine. Typical antigens that trigger IgE responses include allergens and parasitic worms

56
Q

How is the Variable Domain of an Immunoglobulin Heavy or Light Chain Encoded

A

Encoded by more than one gene segment

57
Q

Encoding of Variable Domain of Light Chain

A

Variable domain is encoded by a variable or V gene segment and a shorter joining or J gene segment

58
Q

Encoding of Variable Domain of Heavy Chain

A

Variable domain is composed of a V gene segment, a short diversity (D) gene segment and a J segment

59
Q

Rearrangements To Produce Immunoglobulin Heavy and Light Chains

A

The rearrangements that produce immunoglobulin heavy and light chains involve random selection of individual V, D, and J gene segments

60
Q

Assembly of Light Chain and Heavy Chain

A

Assembly of the light chain involves a single joining step between a V and J gene segment, whereas assembly of the heavy chain involves two joining steps, first D to J and then V to D-J

61
Q

What Follows Gene Rearrangments of Heavy and Light Chains

A

Following gene rearrangement, genes are transcribed and the primary transcript is spliced to remove introns

62
Q

CDR1, CDR2, and CDR3

A

The V region genes encode areas of the heavy and light chains that include the complementary determining regions 1 and 2 (CDR1 and CDR2), whereas the joints between the V, D, and J elements encompass CDR3

63
Q

Why is the CDJ Recombination Process Highly Regulated

A

Highly regulated so that each B cell only expresses a single productively rearranged antibody receptor

64
Q

Where Does Rearrangement Occur First and Second

A

Rearrangement of one allele at the heavy chain occurs first and if this is successful, rearrangement at the light locus is initiated

65
Q

How is the Assembly of Gene Segments Guided

A

Guided by DNA sequences, called recombination signal sequences that flank the individual gene segments

66
Q

What Does Each Recombination Signal Sequence Consist Of

A

Each recombination signal sequence consists of a conserved sequence of 7 nucleotides (a heptamer) and a conserved sequence of 9 nucleotides (a nonamer), separated from each other by a non-conserved spacer sequence of either 12 nucleotides (approximately one turn of a DNA helix) or 23 nucleotides (approximately two turns of a DNA helix)

67
Q

12/23 Rule (Immunoglobulin Production)

A

Recognition sequences containing a 12-nucleotide spacer can only recombine with a recognition sequence containing a 23-nucleotide spacer. Avoids erroneous rearrangment and permits DNA fragments to be rearranged in a highly ordered manner

68
Q

VDJ Recombinase

A

In recombination, VDJ recombinase binds with the recombination sequences and cleaves DNA

69
Q

RAG-1 and RAG-2

A

Two lymphocyte-specific proteins, which are essential for the VDJ recombination process

70
Q

Severe Combined Immune Deficiency (SCID)

A

When human subjects with a genetic disruption in either RAG-1 or RAG-2 genes are unable to undergo VDJ recombination and thus lack mature B and T cells, resulting in SCID

71
Q

Omenn Syndrome

A

On other subject, RAG genes are only partially disrupted, leading to a leaky phenotype, involving the non-physiological expansion of small populations of T cells, leading to inflammatory reactions similar to those seen in graft-versus-host disease (a complication of bone marrow or stem cell transplantation)

72
Q

Imprecision of Joining of Different Gene Segments in Immunoglobulin Production

A

A key process of VDJ recombination. Enzymes can either take away or randomly add nucleotides at the joints. The random addition of nucleotides, often called N-nucleotides, by the enzyme TdT plays a particularly important role. Dramatically increases antigen receptor diversity, but can often result in the generation of non-functional proteins

73
Q

Enzyme Terminal Deoxynucleotidal Transferase (TdT)

A

Is responsible for increasing diversity by either taking away or randomly adding nucleotides at the joints of the heavy and light chains. Addition of N-nucleotides can dramatically increase antigen receptor diversity, but often results in the generation of a non-functional proteins

74
Q

Erroneous Targeting of the VDJ Rearrangement Process

A

Can result in chromosomal translocations, which are present in a large number of B cell (and sometimes T cell) tumors

75
Q

Burkitt Lymphoma

A

Involves a translocation of the immunoglobulin heavy or light chain promoter/enhancer to the myc oncogene, resulting in uncontrolled myc expression. Also involves Epstein-Barr virus infection of B cells, as well as immune suppression, commonly associated with malaria

76
Q

Two Principle Mechanisms That Generate Diversity in Antibody Receptors Prior to Antigen Exposure to B Cells

A

Combinatorial diversity and junctional diversity

77
Q

Combinatorial Diversity

A

There are multiple different copies of distinct gene segments (V, D, and J segements) that can be combined in a random manner. In addition, heavy and light chains can pair in many different combinations

78
Q

Junctional Diversity

A

This is introduced at the joints between the gene segments as a result of removal and addition of nucleotides during the recombination process

79
Q

Regulation of VDJ Recombination and Allelic Exclusion

A

Receptor gene assembly is initiated on only one allele of the heavy chain locus. If this leads to a successful rearrangement, rearrangement of the other heavy chain allele is prevented in a process called allelic exclusion, and rearrangement is initiated at one allele of the k light chain locus. Should rearrangement at the heavy chain locus fail, rearrangement at the other allele will be initiated. If rearrangement at the second heavy chain is also non-successful, further rearrangement is halted and the immature B cell will be targeted for death by apoptosis. Same thing happens with light chains at kappa first and then lambda. Processes are in place in the bone marrow to week out many self-reactive B cells. Additional mechanisms are in place in the periphery to prevent the activation of autoreactive B cells that escaped the weeding out process in the bone marrow

80
Q

High-Avidity Interactions

A

Often occurs with surface receptors, antigens with repeated structures, or soluble antigens expressed at high levels. Such strong interactions lead to apoptosis (clinal deletion) of the autoreactive B Cell. Sometimes, such autoreactive B Cells obtain a second change and re-express the RAG genes to undergo secondary VDJ rearrangements (with deletion of the autoreactive receptor) in the hope of generating a non-autoreactive receptor, in a process called receptor editing. If the new receptor is autoreactive as well, the B Cell will be deleted

81
Q

Intermediate-Avidity Interactions

A

This may occur against most soluble self-proteins. Such B Cells are functionally inactivated or anergized. Such anergic B Cells may leave the bone marrow but do not react against antigens and may eventually die

82
Q

Low-Avidity Interactions

A

If receptors recognize self-antigens present in bone marrow at very low levels, the B cells may survive and be exported to the periphery. Such B Cells are unaware or ignorant of the self-antigen they recognize

83
Q

No Self-Reactivity

A

Such B Cells are exported to the periphery and will differentiate further to full maturity

84
Q

How are B Cells Activated to Secrete Antibodies

A

Some activated B cells will differentiate into plasma cells that secrete large amounts of antibodies. Plasma cells typically migrate to bone marrow or the mucosal immune system, where they might produce antibodies for several weeks. Following infection or vaccination, small numbers of antigen-specific B cells will continuously differentiate into plasma cells even after the pathogen or immunizing antigen has been long cleared. This process is critical for the generation of neutralizing antibodies against many pathogens and the effectiveness of most vaccines

85
Q

How are immunoglobulins switched from the membrane to the secreted form

A

Switching of the immunoglobulins from the membrane to the secreted form is accomplished by differntial RNA processing

86
Q

Somatic Hypermutation

A

Following activation in the presence of T cell help, B cells within germinal centers of peripheral lymphoid organs can undergo point mutations within the variable region gene segments, this is called “somatic hypermutation”

87
Q

Activation-Induced Deaminase (AID)

A

Enzyme responsible for somatic hypermutation. Mutations might result in decreased, similar, or increased affinity of the antibody with its cognate antigen

88
Q

Where do B Cells Expressing Surface Immunoglobulin With Increased Affinity for Antigen Bind

A

B Cells expressing surface immunoglobulin with increased affinity for antigen will bind more avidly with antigen displayed on the surface of follicular dendritic cells in germinal centers and will show a proliferative advantage

89
Q

Affinity Maturation of the Antibody Response

A

Greater avidity of B cells will result in the selective survival of B cells expressing antigen receptors with the highest affinity for antigen (evolution on a microscale), this is known as affinity maturation of the antibody response

90
Q

Repeated Immunizations

A

Because the antigen-binding site of an antibody is formed by CDR1, CDR2, and CDR3 regions within heavy and light chains, repeated immunizations result in enrichment of mutations within the CDRs, germinal center reaction causes antibody resopnses to improve over time

91
Q

First Antigen Receptors Expressed By B Cells

A

IgM and IgD varieties

92
Q

First Antibody Secreted During an Immune Response

A

IgM

93
Q

Class/Isotype Switching

A

Later in the immune response, antibodies with the identical antigen-specificity (i.e the same heavy and light chain variable domains) may be of the IgG, IgA, or IgE variety, known as class or isotype switching

94
Q

Requirements for Class/Isotype Switching

A

Requires prior B cell activation and T cell help

95
Q

Switch Region of DNA/Class Switch Recombination/Selection of the Switch Region

A

At the molecular level, each immunoglobulin heavy chain constant region gene fragment is preceded by a stretch of repetitive DNA, called a switch region, that guides a recombination process, called a class switch recombination. When a B cell switches from the co-expression of IgM and IgD to the expression of another isotype, DNA recombination occurs between the switch region upstream of Ig-mu to another Ig gene. AID, involved in somatic hypermutation, also plays a critical role in class switch recombination. Selection of the switch region is guided by cytokines produced by other immune cells, in particular antigen-experienced, CD4-expressing T lymphocytes

96
Q

Number of Rounds of Class Switching Recombination B-Cells Might Undergo

A

Individual B Cells might undergo more than one round of class switch recombination. T cells often produce a mixture of cytokines, resulting in divergent class switching in individual B Cells with the same antigen specificity